An example of a microwave conductive structure is a microwave susceptor which is an article which absorbs microwave energy, converts it into heat and conducts the heat generated into food articles placed in close proximity thereto. Microwave susceptors are particularly useful in microwave food packaging to aid in browning or crisping those foods which are preferably prepared in that way.
The field of microwave conductive packaging technology includes numerous attempts to optimize heating, browning and crisping of food cooked in microwave ovens. Such attempts include the selectively microwave-permeable membrane susceptor shown in prior U.S. Pat. No. 5,185,506, issued Feb. 9, 1993 and U.S. Pat. No. 5,245,821 issued Oct. 19, 1993. Other attempts include a microwaveable barrier film described in U.S. Pat. No. 5,256,846 issued Oct. 26, 1993 and a microwave diffuser film described in U.S. patent application Ser. No. 07/756,165. U.S. Pat. Nos. 5,185,506 and 5,245,821 disclose examples of constructions which modify the overall heating pattern in a microwave oven in an attempt to optimize the heating for a specific food product and geometry. However, these and conventional microwave susceptor structures do not adequately address the heating problems associated with non-uniform electromagnetic fields found in all microwave ovens.
The unpredictability of the microwave field within a microwave oven is a significant problem for articles and methods which attempt to make heating, browning or crisping of food uniform. There are more than 500 models of microwave ovens on the market today, all of which have different heating patterns and non-uniform energy fields. Since most food products themselves are non-uniform in size and shape, there is an increased natural tendency of food to heat unevenly. The inability to adequately predict locations of hot spots and cold spots within a microwaved, packaged food item including a susceptor has made this area the subject of much research. For example, fishsticks or french fries loosely packaged in a box containing a six-inch by six-inch susceptor on the bottom, are often not properly crisped. After exposure to the microwave field in a microwave oven, there will be noticeable differences in the heat generated by the 36-inch square susceptor, depending on the location of the food product. For instance, wherever the food product does not cover the susceptor material, the susceptor will get extremely hot, often hot enough to cause damage to the package. Indeed, it has been reported that susceptor packages have caught fire in consumer microwave often. On the edges of the food product, there will also be extremely high temperatures relative to the center of the food product. However, on the edges of the food product, there will be lower temperatures than those susceptor areas which are not covered by food product. The net result is that the heat gain of the susceptor is not balanced over the susceptor area.
Therefore, one goal of the present invention is to provide a microwave conductive structure which exhibits enhanced safety and performance over existing commercial microwave susceptors, and a second goal is to provide a microwave conductive structure which adapts itself in a controlled manner on the basis of the oven, food geometry, food location and food composition, so as to provide more uniform heating, browning and crisping of food products.